This invention relates to cryostats for the containment of very low temperature liquids such as liquid helium and in particular to cryostats housing superconducting apparatus such as NMR spectrometer magnets.
Prior art cryostats for containment of superconducting apparatus, for example, superconducting magnets, have employed a helium vessel shaped to exhibit a relatively small cylindrical volume surrounding the superconducting magnet, in open communication with a larger volume disposed immediately above the solenoid. In this geometry the solenoid is maintained completely submerged in the liquid helium bath. A sufficient hold time for the liquid helium is provided by the head of liquid helium in the large volume. This form of helium reservoir exhibits a surface area to volume ratio substantially higher than the minimum achievable; consequently additional radiation losses are introduced which contribute to a higher rate of helium boil-off.
Prior art cryostats have taken the form of nested chambers which have been internally braced, as for example with stainless steel spokes, to withstand mechanical shock and to maintain minimum clearances between adjacent nested walls. Stainless steel has been a popular material of choice because of its relatively low thermal conductivity and its high strength. However, the thermal conductivity of such bracing places a limit upon the thermal isolation which can be achieved between adjacent surfaces of nested structures.
Cryostats of the prior art have employed a secondary temperature bath to shield the lowest temperature coolant from ambient temperature. Ordinarily, the secondary coolant reservoir is itself insulated from ambient temperature, as for example with layers of an insulating material. In a superconducting magnet with room temperature access, a relatively large magnet bore is required by prior art cryostat structures to provide sufficient space for this insulation. As a result, the inside diameter of such prior art solenoid is constrained to span a proportionately larger diameter to accomodate the additional insulation, whereby a much greater length of superconducting wire is required for fabrication of the solenoid.